The present invention relates to a feed forward amplifier. Particularly, the present invention relates to a technique of being capable of improving the distortion removing effect by feed forward even when non-linear distortion of a main amplifier is compensated using an adaptive pre-distortion compensation method.
Recently, various feed-forward amplifiers are being developed. For example, according to the technique disclosed in JP-A-No. 284951/1998, the feed-forward amplifier includes a pre-distortion compensation circuit 3, a variable attenuator 4, a variable phase shifter 5, and a main amplifier 6, arranged in the listed order. The control circuit 19 controls the variable attenuator 4 and the variable phase shifter 5 to minimize the carrier component, or an input signal, in detected distortion components. The technique disclosed in the above publication can compensate distortions even if there are temperature variations.
A pre-distorter (pre-distortion compensator) is disposed on the main amplifier side of a distortion detection loop to relieve the burden of the auxiliary amplifier used in a feed-forward amplifier. The use of an adaptive pre-distorter (adaptive pre-distortion compensator) in an adaptive pre-distortion system, which can reduce non-linear distortion more than the pre-distorter, is being developed. The adaptive pre-distortion system is one of compensating non-linear distortions produced in amplifiers. According to this system, the characteristics reversed to characteristics of an amplifier are provided to an input signal. Thus, this pre-distortion system, which linearly operates an amplifier apparently, can adaptively deal with changes in characteristic of the amplifier due to changes in ambient environment.
However, there is the problem that when the adaptive pre-distorter is used to compensate non-linear distortion of the main amplifier, the vector modulator controlled by the adaptive pre-distorter may rather deteriorate the distortion decreasing effect by the feed forward.